When drilling or completing wells in earth formations, various fluids typically are used in the well for a variety of reasons. Common uses for well fluids include: lubrication and cooling of drill bit cutting surfaces while drilling generally or drilling-in (i.e., drilling in a targeted petroliferous formation), transportation of “cuttings” (pieces of formation dislodged by the cutting action of the teeth on a drill bit) or debris to the surface, controlling formation fluid pressure to prevent blowouts, maintaining wellbore stability, suspending solids in the well, minimizing fluid loss into and stabilizing the formation through which the well is being drilled, fracturing the formation in the vicinity of the well, displacing the fluid within the well with another fluid, cleaning the well, testing the well, transmitting hydraulic horsepower to the drill bit, fluid used to place a packer, abandoning the well or preparing the well for abandonment, and otherwise treating or servicing the well or the formation.
During some drilling operations, the pressure differential between the circulating wellbore fluids and that of the formation being drilled may be maintained in an underbalanced mode. Underbalanced drilling is a method of drilling a desired subterranean formation where the hydrostatic pressure exerted by a column of wellbore fluid in the drill string is less than the natural pressure (pore pressure) inherent in the subterranean formation being drilled. Underbalanced drilling may prevent damage to the desired subterranean formation and in particular low pressure formations. Typically, the pressure differential is set to provide a margin above the pressure at which wellbore collapse might occur. The introduction of sufficient air, nitrogen or other gases to the wellbore fluids has been used to reduce the density of the commingled fluids and effectively decrease hydrostatic pressure. Other low density fluids such as emulsions, foams and mists have been used as a wellbore fluid to achieve an underbalanced condition.
Coiled tubing drilling may provide certain advantages when drilling or servicing a well in an underbalanced mode, such as a smaller rig footprint, less waste disposal, lower energy consumption, easier tripping in and out, as well as other advantages known to those in the art. The use of coiled tubing, which is a relatively thin walled strip of sheet metal coiled and edge-welded into a continuous tube which is able to transmit a longitudinal thrust force while being flexible enough to be wound onto a drum or passed around a bend, has been known in drilling operations for some time. Coiled tubing operations were originally developed for workovers (treatment, re-stimulation, and maintenance) of existing oil and gas wells. The continuous roll of tubing allowed for rapid insertion and retraction of downhole tools, and enabled these operations to be completed without the need for a conventional workover rig. Coiled tubing drilling (CTD) has been in use for some time, typically for the placement of substantially vertical, slim hole wells (typically gas wells), although CTD technology may be more recently used for deep directional and horizontal wells, as well as horizontal re-entry drilling.